Algorithms for string similarities (better than Levenshtein, and similar_text)? Php, Js

Here's a solution that I've come up to. It's based on Tim's suggestion of comparing the order of subsequent charachters. Some results:

• jonas / jonax : 0.8
• jonas / sjona : 0.68
• jonas / sjonas : 0.66
• jonas / asjon : 0.52
• jonas / xxjon : 0.36

I'm sure i isn't perfect, and that it could be optimized, but nevertheless it seems to produce the results that I'm after... One weak spot is that when strings have different length, it produces different result when the values are swapped...

static public function string_compare($str_a, $str_b) 
{
    $length = strlen($str_a);
    $length_b = strlen($str_b);

    $i = 0;
    $segmentcount = 0;
    $segmentsinfo = array();
    $segment = '';
    while ($i < $length) 
    {
        $char = substr($str_a, $i, 1);
        if (strpos($str_b, $char) !== FALSE) 
        {               
            $segment = $segment.$char;
            if (strpos($str_b, $segment) !== FALSE) 
            {
                $segmentpos_a = $i - strlen($segment) + 1;
                $segmentpos_b = strpos($str_b, $segment);
                $positiondiff = abs($segmentpos_a - $segmentpos_b);
                $posfactor = ($length - $positiondiff) / $length_b; // <-- ?
                $lengthfactor = strlen($segment)/$length;
                $segmentsinfo[$segmentcount] = array( 'segment' => $segment, 'score' => ($posfactor * $lengthfactor));
            } 
            else 
            {
                $segment = '';
                $i--;
                $segmentcount++;
            } 
        } 
        else 
        {
            $segment = '';
            $segmentcount++;
        }
        $i++;
    }   

    // PHP 5.3 lambda in array_map      
    $totalscore = array_sum(array_map(function($v) { return $v['score'];  }, $segmentsinfo));
    return $totalscore;     
}

In addition to levenshtein() and similar_text(), there's also:

soundex(): Returns the four-character soundex key of a word, which should be the same as the key for any similar-sounding word.
metaphone(): Similar to soundex, and possibly more effective for you. It's more accurate than soundex() as it knows the basic rules of English pronunciation. The metaphone generated keys are of variable length.

Please, be careful about using string_compare :

ivanov ivan / ivanov ivan : 1 OK!

ivanov ivan2 / ivanov ivan : 1 o_O

ivanov ivan / ivanov i : 1.1363636363636 OMG!